In the days since more than 120 people were killed in a terrorist attack in Paris, conservative presidential candidates have called for limiting the number of Syrian refugees allowed into the U.S. The Fix's Amber Phillips breaks this down. (Sarah Parnass/The Washington Post)

Police forensics investigators work at the crime scene of a mass shooting at the Pulse gay night club in Orlando, Florida, U.S. June 12, 2016. REUTERS/Carlo Allegri

Following the deadly terrorist attacks in Lebanon and Paris last week, international terrorism has again taken center stage. The Islamic State has claimed responsibility for both attacks, and has called them “the first of the storm.” In a new video released Monday, the group has threatened similar attacks on those conducting airstrikes in Syria, singling out Washington, D.C.

For the past 10 years, we have researched the connection between terrorist threat and public opinion. In our book, “Democracy at Risk,” and in our more recent work, we argue that public attitudes and evaluations shift in at least three politically relevant ways when terrorist threat is more prominent in the news. By understanding these tendencies, we can anticipate how the American public may react to the Paris attacks, and what this may mean for the 2016 presidential campaign.

First, when terrorist threat is pronounced, individuals become less trusting of others, even their own neighbors. They express less favorable attitudes toward immigrants. In particular, they become less supportive of the rights of Arab and Muslim Americans.

It is not surprising, then, that since the Paris attacks, many politicians have made statements that reflect distrust and express concern about immigrants and Muslims. After news that at least one of the Paris suspects entered with a wave of Syrian refugees, some Republican presidential candidates called for the U.S. to let in only Christian (that is, non-Muslim) refugees from Syria. Governors from many states expressed opposition to the resettlement of Syrian refugees in their states. Donald Trump said that he would consider closing some mosques.

In short, concerns about terrorism increase suspicion and even intolerance directed at migrants, refugees and Muslims.

Second, terrorist threats help increase the public’s support for certain political leaders. The public’s tendency to rally around a sitting executive when confronted with an external threat is well documented. But we find the public does more than that: It projects leadership qualities onto political figures and then it weights leadership more heavily when voting on Election Day. But not all political leaders are affected in the same way. On average, leaders who are Republican, male, and have relevant national security experience tend to be viewed as more competent.

In another study we conducted with Mirya Holman in 2005, we found that evaluations of Hillary Clinton decreased when terrorism was made salient, but evaluations of John Kerry remained the same and evaluations of George Bush and Condoleezza Rice became more favorable. Leaders who are both female and Democratic may therefore experience the most negative political consequences of terrorist attacks. (Of course, this study was conducted before Clinton served as secretary of state, which we revisit below.)

A third way public opinion shifts in response to terrorism is toward support for more hawkish policies in foreign affairs and homeland security, even at the expense of civil liberties. This hawkishness occurs among partisans of all stripes.

So what should we expect for the presidential campaign under way? In the Democratic primary, we would not expect major changes in support for Clinton or Sanders. Although the public may evaluate Democratic women less favorably when terrorist threat is salient, as of 2015, Hillary Clinton is likely to be bolstered by the foreign policy experience she gained as secretary of state and by her tendency to take stands that are more hawkish than those of Bernie Sanders.

The increased salience of terrorism may prove more consequential for the Republican nomination. Candidates who have taken more isolationist stands on foreign policy, such as Rand Paul, may be perceived as less attractive, while those who have taken more hawkish stands — such as Marco Rubio, Ted Cruz and Jeb Bush — may be more advantaged. Primary voters may also be less likely to support candidates with little experience in government and thus national security, such as Ben Carson and Carly Fiorina.

Trump is a bit of a wild card, however. His bold style may be appealing in a context of threat. However, he may be hurt by his lack of any significant foreign policy experience, a fact he himself recognized during the Republican foreign policy debate.

The events of this past week are horrific and terrifying. Our research shows how individuals often react to heightened terrorist threat in how they treat others, evaluate leaders and make decisions on Election Day. Some of these reactions are natural reactions to concerns about terrorism, but our research suggests that some of this reaction may be driven more by fear than by reason.

Jennifer L. Merolla is professor of political science at the University of California, Riverside. Elizabeth J. Zechmeister is professor of political science, Vanderbilt University. They are the authors of “Democracy at Risk: How Terrorist Threats Affect the Public” (University of Chicago Press, 2009).